Dynamic Simulation of Debris Bed Melting Process during the Hypothetical Severe Accident of HPR1000

At the late-phase of nuclear reactor severe accident, the melting process of debris bed in the reactor pressure vessel (RPV) lower head has significant impact on internal heat transfer characteristics, heat flux distribution on vessel wall and vessel wall ablation. In this study, based on the Ansys Fluent, the phase change model and large eddy simulation (LES) turbulence model were used to study the dynamic melting process of debris bed during the hypothetical severe accident of Hua-long pressurized reactor 1000 (HPR1000). The variations of temperature distribution, velocity field and wall ablation during the molten pool formation were predicted. The results showed that the heating rate decreased and tended to be stable after the melting of the debris bed began. The temperature distribution in the pool gradually became relatively uniform in the middle and upper parts, with a large temperature gradient at the bottom. With the increase of the decay heat, the pool part with uniform temperature expanded downward. Although the heat flux distribution on the wall inside was lower than the critical heat flux (CHF) at the corresponding outer position, wall ablation was still observed. The ablation first occurred on the inside of the wall near the surface of the debris bed and gradually spread downward. The area and depth of the ablation increased with the shortening of the debris dry-out time since reactor shutdown. The calculation results here can provide reference for the study of phase change heat transfer in the debris bed and the integrity of the RPV.